Wireless or mobile communications networks in which a mobile terminal (UE, such as a mobile handset) communicates via a radio link to a network of base stations (eNBs) or other wireless access points connected to a telecommunications network, have undergone rapid development through a number of generations. The initial deployment of systems using analogue signalling has been superseded by second generation (2G) digital systems such as Global System for Mobile communications (GSM), which typically use a radio access technology known as GSM Enhanced Data rates for GSM Evolution Radio Access (GERA), combined with an improved core network.
Second generation systems have themselves been replaced by or augmented by third generation (3G) digital systems such as the Universal Mobile Telecommunications System (UMTS), which uses a Universal Terrestrial Radio Access Network (UTRAN) radio access technology and a similar core network to GSM. UMTS is specified in standards produced by 3GPP. Third generation standards provide for a greater throughput of data than is provided by second generation systems. This trend is continued with the move towards fourth generation (4G) systems.
3GPP design, specify and standardise technologies for mobile (cellular) wireless communications networks. Specifically 3GPP produces a series of technical reports (TR) and technical specifications (TS) that define 3GPP technologies. The focus of 3GPP is currently the specification of standards beyond 3G, and in particular an Evolved Packet System (EPS) offering enhancements over 3G networks, including higher data rates. The set of specifications for the EPS comprises two work items: Systems Architecture Evolution (SAE, concerning the core network) and LTE concerning the air interface. The first set of EPS specifications were released as 3GPP Release 8 in December 2008. LTE uses an improved radio access technology known as Evolved UTRAN (E-UTRAN), which offers potentially greater capacity and additional features compared with previous standards. SAE provides an improved core network technology referred to as the Evolved Packet Core (EPC). Despite LTE strictly referring only to the air interface, LTE is commonly used to refer to the whole of the EPS, including by 3GPP themselves. LTE is used in this sense in the remainder of this specification, including when referring to LTE enhancements, such as LTE Advanced. LTE is an evolution of the UMTS and shares certain high level components and protocols with UMTS. LTE Advanced offers still higher data rates compared to LTE and is defined by 3GPP standards releases up to and including 3GPP Release 12. LTE Advanced is considered to be a 4G mobile communication system by the International Telecommunication Union (ITU).
Embodiments of the present invention are implemented within an LTE mobile network. Therefore, an overview of an LTE network is shown in FIG. 1. The LTE system comprises three high level components: at least one UE 102, the E-UTRAN 104 and the EPC 106. The EPC 106 communicates with Packet Data Networks (PDNs) and servers 108 in the outside world. FIG. 1 shows the key component parts of the EPC 106. It will be appreciated that FIG. 1 is a simplification and a typical implementation of LTE will include further components. In FIG. 1 interfaces between different parts of the LTE system are shown. The double ended arrow indicates the air interface between the UE 102 and the E-UTRAN 104. For the remaining interfaces media is represented by solid lines and signalling is represented by dashed lines.
The E-UTRAN 104 comprises a single type of component: an eNB which is responsible for handling radio communications between the UE 102 and the EPC 106 across the air interface. An eNB controls UEs 102 in one or more cell. A cell may refer to the area of reception defined by a single antenna. However, where an eNB implements downlink CoMP there may be multiple transmission points used within a cell. From the perspective of the UE in some deployment scenarios the transmissions may be seen as resulting from a single cell. LTE is a cellular system in which the eNBs provide coverage over one or more cells. Typically there is a plurality of eNBs within an LTE system. In general, a UE in LTE communicates with one eNB through one cell at a time. From 3GPP Release 10 an eNB may configure the UE with multiple serving cells, one on each serving frequency, which is known as Carrier Aggregation. The eNB itself typically includes a tower with one or more antennas for transmitting and receiving wireless signals to and from a UE.
Key components of the EPC 106 are shown in FIG. 1. It will be appreciated that in an LTE network there may be more than one of each component according to the number of UEs 102, the geographical area of the network and the volume of data to be transported across the network. Data traffic is passed between each eNB and a corresponding Serving Gateway (S-GW) 110 which routes data between the eNB and a PDN Gateway (P-GW) 112. The P-GW 112 is responsible for connecting a UE to one or more servers or PDNs 108 in the outside world. The Mobility Management Entity (MME) 114 controls the high-level operation of the UE 102 through signalling messages exchanged with the UE 102 through the E-UTRAN 104. Each UE is registered with a single MME. There is no direct signalling pathway between the MME 114 and the UE 102 (communication with the UE 102 being across the air interface via the E-UTRAN 104). Signalling messages between the MME 114 and the UE 102 comprise EPS Session Management (ESM) protocol messages controlling the flow of data from the UE to the outside world and EPS Mobility Management (EMM) protocol messages controlling the rerouting of signalling and data flows when the UE 102 moves between eNBs within the E-UTRAN. The MME 114 exchanges signalling traffic with the S-GW 110 to assist with routing data traffic. The MME 114 also communicates with a Home Subscriber Server (HSS) 116 which stores information about users registered with the network.
The UE can exist in one of two communication states in LTE: an idle state (Radio Resource Control Idle, RRC-IDLE) in which the UE is basically on standby, and a connected state (RRC-CONNECTED) in which the UE has an active radio link to the eNB.
In the RRC-IDLE state in LTE, the UE is tracked by the network to a specific tracking area, which may cover several eNBs. The UE is not aware of the network topology. Rather the UE is only aware of logical entities (for instance cells and tracking areas) but does not know which cell is connected to which eNB. The UE may choose which cell to listen to. The main aim in this state is to minimise signalling and resources, and thereby maximise standby time for terminals with limited battery power.
In contrast, in the RRC-CONNECTED state in LTE, the UE has a serving eNB allocated to it, has its location tracked to the serving eNB, and has active bearers which allow the terminal to transmit and receive at relatively high data rates and the network chooses the cell(s) by which it serves the UE.
In a recent development of LTE known as coordinated multi-point transmission (CoMP), the eNB is able to configure multiple Transmission Points (TPs) per serving frequency which are geographically remote from each other. Each transmission point may consist of a set of geographically co-located transmit antennas. For downlink in CoMP, the E-UTRAN coordinates the signals transmitted from the different TPs to increase signal strength or to reduce co-channel interference as perceived by the UE. The transmissions from the multiple TPs may be perceived by the UE as resulting from a single cell.
Physical signals are used in LTE for both uplink and downlink in order to support physical layer operations such as channel estimation, scheduling, and synchronisation. One subset of the physical signals is reference signals, which are used, among other things, to enable feedback from the UE on various channels used by the E-UTRAN. For example, Cell-specific Reference Symbols (CRS) can be used by a UE to provide estimates of phase and amplitude of transmissions from different antennas of an eNB. These estimates are fed back to the eNB in real-time for optimisation of transmissions. CRSs are also measured by the UE in order to establish power and quality indicators for the channel. These measurement results (indicators) can be reported back in a measurement report to the network for mobility management and network optimisation.
In CoMP, the transmission points (TPs) can be monitored and measured using a different set of reference signals known as Channel State Information Reference Signals (CSI-RS). Each configuration of a channel state indicator reference signal (CSI-RS) is known as a CSI-RS resource configuration. Although it is possible to configure a CSI-RS resource configuration per transmission point, the LTE standards also allow the eNB to configure a single CSI-RS resource configuration for multiple transmission points.